1. Field of the Invention
This invention relates to a hybrid vehicle provided with an engine and a motor or a motor-generator, as drive power sources, and configured to be able to disengage the engine from the drive power transmission system or motor.
2. Description of Related Art
In a hybrid vehicle that is provided with both a motor or motor-generator (below these may be referred to jointly as “motor”) and an engine as sources of motive power, various advantages are obtained in that it is possible to halt the engine (perform an idling-stop) when the vehicle is stationary, regeneration of energy can be performed by the motor when decelerating, and the engine can be operated at an operating point of good energy efficiency, and so on. In particular, when the time during which the vehicle travels by the power of the motor becomes longer, the effect of improving the overall fuel consumption of the vehicle is raised. This is because, in the case of motor-powered travel, the engine is stopped and fuel is not consumed. In this case, in order to reduce the loss of motive power due to turning over of the engine, it is desirable for the engine to be disengaged from the motor which is outputting drive power for travel or from a power transmission system which transmits this drive power to the vehicle wheels. If a clutch is provided in order to disengage the engine in this way, it is possible to select an operational mode in which the engine is stopped or the engine is driven, during travel by the power of the motor. A hybrid vehicle provided with a clutch for disengaging the engine in this way is described in Japanese Patent Application Publication No. 08-295140 (JP 08-295140 A).
To give a brief description of this composition, an electric generator is coupled to a first rotating element in a differential mechanism provided with three rotating elements, such as a planetary gear mechanism, and furthermore a second rotating element is taken as an output element and a third rotating element is coupled to braking means. The engine is coupled to the third rotating element via a clutch. Furthermore, a motor is coupled to the second rotating element. Consequently, in the composition described in JP 08-295140 A, since the differential mechanism functions as a decelerating device or an accelerating device due to the third rotating element being fixed by the engine or the braking means, then it is possible to cause the electric generator coupled to the first rotating element to function as a motor and to transmit the torque thereof to the output element. In other words, it is possible to cause the vehicle to travel by motive power outputted from the electric generator and the motor. If a unidirectional clutch which fixes the third rotating element by engaging therewith when a torque is applied in a direction causing reverse rotation of the third rotating element is used as braking means, then the third rotating element can be fixed by the unidirectional clutch, independently of the engine, during motor-powered travel, and since the clutch can be disengaged from the third rotating element, then it is possible to continue to drive the engine or to stop the engine.
Furthermore, Japanese Patent Application Publication No. 2013-18399 (JP 2013-18399 A) describes a control apparatus designed for a hybrid vehicle which couples an engine, a motor and a transmission apparatus by interposing a clutch respectively therebetween. JP 08-295140 A does not describe the concrete control relating to stopping and restarting of the engine, whereas the apparatus disclosed in JP 2013-18399 A is configured so as to stop combustion of the engine by a braking operation during travel, and to raise the number of revolutions of the engine by disengagement of the braking operation. Control for raising the engine number of revolutions is performed by both the motive power of the vehicle wheels and the motive power of the motor-generator.
Moreover, Japanese Patent Application Publication No. 2005-162142 (JP 2005-162142 A) describes a method for starting an engine in a hybrid vehicle, and according to this description, when the engine is started during electric-powered travel using the motor as a source of motive power, the clutch provided in a hybrid transmission apparatus including an engine and a motor is engaged gradually, and the engine is gradually cranked as the torque of this clutch is gradually increased. The torque variation during this starting of the engine is absorbed by slipping of the clutch.
Japanese Patent Application Publication No. 2013-86649 (JP 2013-86649 A) describes an apparatus which is configured so as to stop a supply of fuel to an engine when a hybrid vehicle is decelerating, and to delay decline in the number of revolutions of the engine by controlling the speed ratio of a belt-type step-less transmission mechanism which is coupled to the output side of the engine, to a low speed ratio. This control of the reduction in the decline of the engine number of revolutions is control for achieving a restart of the engine by raising the possibility or frequency of the engine rotating, when a request for restarting of the engine is established.